Roving is produced from slivers which are usually pretreated (for example doubled) by drafting and serves as a feed for the subsequent spinning process, in which the individual fibers of the roving are spun, for example by means of a ring spinning machine, to form a yarn. In order to give the roving the strength necessary for the further processing, it has proven to be advantageous, during production of the roving, to draft the supplied fiber bundle by means of a drafting system, which is usually part of the spinning preparation machine in question, and then to provide it with a protective twist. The strength is important in order to prevent breaking of the roving during the winding onto a tube and/or during the feeding thereof to the downstream spinning machine. The applied protective twist must, on the one hand, be strong enough to ensure that a cohesion of the individual fibers during the individual winding and unwinding processes and corresponding transport processes between the respective types of machine is ensured. On the other hand, it must also be ensured that, despite the protective twist, the roving can be further processed in a spinning machine—the roving must therefore still be able to be drafted.
For producing such a roving, in the past so-called flyers were used, the delivery speed of which is nevertheless limited due to centrifugal forces that occur. There have therefore already been many proposals to avoid the flyers or to replace them with an alternative type of machine.
In this connection, it has also already been proposed, inter alia, to produce roving by means of air-jet spinning machines, in which the protective twist is created by means of swirled air flows. The basic principle here consists in guiding a fiber bundle through a consolidating means designed as an air spinning nozzle, in which an air vortex is generated. The latter finally effects that some of the outer fibers of the supplied fiber bundle are wrapped as so-called wrapping fibers around the centrally running fiber strand, which in turn consists of core fibers running substantially parallel to one another.
Another method for roving production is disclosed in DE 24 47 715 A1. The consolidation of the unconsolidated fiber bundle described therein takes place by a consolidating means which brings about not a twisting, but rather a helical wrapping of a sliver with one or more filament yarns, preferably monofilament yarns, which hold the fiber bundle together and give it its strength. The spirals of the individual filament yarns may in this case be arranged in the same direction or in opposite directions. Preference is given to two filament yarns which are arranged in opposite directions of rotation and in a manner crossing over one another. The roving produced in this way is thus composed essentially of a sliver of parallel staple fibers and one or more fine-titer filament yarns wrapping helically around the sliver.
There are various possibilities for wrapping the filament yarn or filament yarns around the unconsolidated fiber bundle. For example, the filament yarn can be applied onto small bobbins of small diameter. The filament yarn is then drawn off from the stationary bobbin and drawn through the bobbin axis together with the fiber bundle, whereby the filament yarn is wrapped around the fiber bundle and the number of windings drawn off from the bobbin corresponds to the number of wraps applied to the fiber bundle. In principle, it is also possible to design the consolidating means in such a way that only the unconsolidated fiber bundle is guided through the bobbin axis, so as to hereby relocate the winding process to behind the filament yarn bobbin. The wrapping point should in this case be defined by a suitable thread guide.
Another method for producing roving is described in WO 2009/086646 A1, wherein the method comprises the following steps: 1) providing a fiber bundle in the form of two, preferably untwisted, slivers, 2) applying S and Z twists over alternating regions of the two slivers, wherein regions of S and Z twists on the respective sliver are separated by regions without any twist, 3) bringing together the two slivers provided with S and Z twists to form a roving, wherein the two slivers automatically twist together on account of their tendency to twist back.
The S and Z twists may be created for example by means of two elements of the consolidating means used, which hold the respective sliver in a clamped manner, wherein at least one element, preferably both elements, apply opposite twists on the sliver in an alternating manner on both sides by a relative movement on the surface thereof transversely to the longitudinal direction of the sliver. At the same time, the respective sliver is moved in the sliver direction. However, the S and Z twists can also be created by means of an aerodynamic, in particular pneumatic, method.
The alternating S and Z twists are moreover interrupted by intermediate regions without any twist. The two slivers provided with S and Z twists in the same way are finally brought together at the so-called joining point. Here, the slivers start to twist together automatically, that is to say they wind around each other. This so-called double-folding maintains the S and Z twists in the individual slivers, so that a self-stabilizing two-component roving is obtained. In principle, however, care should be taken here to ensure that the regions without any twist in the first sliver should be arranged offset in the longitudinal direction relative to the regions without any twist in the second sliver, so that two regions without any twist in the first and second sliver never lie next to one another in the resulting roving, since the strength of the roving depends substantially on the phase position of the regions without any twist in the two slivers. As described above, the rovings are therefore always brought together by the consolidating means in such a way that their regions without any twist lie out of phase. The roving produced in this way ultimately has a greater strength than an untwisted fiber bundle, said strength ultimately being sufficient to wind the roving onto a bobbin and unwind it again from the latter without false drafts.
However, if there are interruptions in the roving production process, for instance because the roving between the consolidating means and the winding device breaks or because the consolidating means is clogged up, the roving that is already wound onto the tube must be joined to the roving produced by the consolidating means after the latter is restarted, since the tube should be loaded with one continuous roving strand. This is time-consuming, however, and therefore affects the productivity of the textile machine in question.